Yep, did the same for me. Wanted to understand why WeeWX-WD never had the problem, first thought was what user was running wee_extension, dead end. Then I wondered if the json sub-directory was doing it. I had a quick look at the extension.py code, I hadn't come across os.path.commonprefix() before so googled it. os.path.commonprefix() checks a list of paths and returns the longest common prefix and it does this by doing a character-by-character comparison, when I saw the warning that it could return invalid paths that peaked my curiosity. In the case of your extension /home/weewx/skins/Belchertown is the result, so extension.py then works from /home/weewx/skins/Belchertown down and thus /home/weewx/skins/Belchertown_Highcharts is ignored. I did some rejigging of your extension so that it used /home/weewx/skins/ZBelchertown_Highcharts instead and it worked fine. That explains why WeeWX-WD was never affected, all of its skin directory names had no common prefix. Probably more correctly (and I didn't check this) but the fact that the common prefix includes the name of one of the skin directories (Belchertown) may be the cause.
No matter, its a bug that needs to be fixed. Can't comment on the proposed fix, better for Tom or Matthew I think. Gary
